In order to increase the power reserve, by increasing the number of turns of a mainspring, one solution consists in decreasing the diameter of the barrel arbour and of the associated core, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be too small; there is a risk of breakage if the core diameter is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbour or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step joined to this pivot shoulder limits the axial clearance of the arbour or of the core relative to a bottom plate or to a bridge carrying a jewel or similar element.
An even larger diameter than that of the step is required for a shoulder pivotally guiding the drum on the arbour or on the core, combined with a step limiting the axial clearance of the drum. The dimension chain required to observe minimal sections of material results in substantial dimensions, which are difficult to reduce. In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
CH Patent No 319631 in the name of FELSA SA discloses a double barrel, and the manner in which the clearance of each drum is limited. The clearance of the bottom drum is limited on the top side by the collar of a sleeve, which is pushed towards a shoulder of the barrel arbour, by a nut screwed onto the arbour; and, on the bottom side, by the base of a core pivoting integrally with the barrel arbour via a flat portion. The length of the bottom shoulder of the barrel arbour and that of the drum pipe determine the drum clearance, in relation to these collar and base surfaces.
U.S. Pat. No. 1,110,061 in the name of KIENZLE discloses a core detachably mounted on a barrel arbour (in the direction of pivoting); the core is supported both on the plate and the bridge, while the two shoulders of the core limit the travel of the drum and of the cover.
U.S. Pat. No. 3,564,839 in the name of WUTHRICH discloses a barrel which includes a screw thread on the bridge for supporting the drum, and a drum having a pipe. The cover is a simple metal plate placed on the bottom plate, with no connection to the arbour or to the drum. The arbour includes two shoulders, one resting on the core, the other intended to cooperate in abutment with the drum pipe.
GB Patent No 1162296 in the name of CITIZEN discloses a spring of variable thickness, but there is no teaching concerning the height features of the spring.
CH Patent No 341764 in the name of AUGE discloses a spring with strips of different widths, welded to each other.
All of these documents disclose barrel arbours with a hook driving the spring, but none with a pivoting barrel arbour for receiving the spring. The documents do not particularly describe any recess, on the arbour or the core, in which the spring could be positioned. Some Patent documents disclose springs whose rigidity is modified on the inner end coil, by modifying its thickness or otherwise; the geometry of the spring is not taken into account for centring the spring.